are any of the different conduit sizes stronger than each other?
It’s the thickness of the tube/conduit wall that affects strength the most.
Diameter.
Vague questions are hard to answer. Get the size available in your country. You are asking questions with no way for us to really help you. The only reason there are three sizes is globally.
As for spindles, even if you are ordering online we don’t know what country you are plugging it into.
OH sorry, im in america. I meant the 3 sizes (23.5, 25mm, 25.4mm) Or do they just use a different one in different countries like they do power outlets?
Exactly. Here we have 3/4"EMT has a standard 23.5mm OD. You might be able to get other sizes but it would only be from a metal supplier and significantly more expensive.
Oh thanks